This invention relates to a method for providing a substrate with a layer of a tertiary compound comprising silicon, silicon carbide and a transition series metal. The thus coated substrate may or may not have an additional top layer of silicon carbide adhered to said tertiary compound layer.
Substrates coated in accordance with the invention demonstrate excellent resistance to corrosive attack, and also demonstrate reasonable electronic conductivity. Thus, the coated substrates are well suited for use in the preparation of alkali metal/polysulfide batteries wherein the substrate is exposed to corrosive attack by molten polysulfide salts.
The method of the invention allows the coating of a substrate not only with tertiary compounds, but alternatively with or without silicon carbide surface layers in an economical and simple manner. The tertiary compounds which are applied as a layer to the substrate in accordance with this invention have previously usually been formed by dissolving silicon carbide in the melt of a transition metal. This method of preparing transition series metal tertiary compounds, however, requires temperatures in the order of 1500.degree. C. or higher. These temperatures are above the melting temperatures of many substrates, including steels, and thus make prior art methods of manufacturing these tertiary compounds unacceptable for application to substrates.
Prior art preparation of silicon carbide/transition series metal materials is described in Pellegrini & Feldman, "LPE Growth of SiC Using Transition Metal-Silicon Solvents", Proceedings of Third International Conference on Silicon Carbide, University of South Carolina Press, 1973; Wolff, Das, Lamport, Mlavski & Trickett, "Principles of Solution and Travelling Solvent Growth of Silicon Carbide", Material Research Bulletin, Vol. 4, pages S-67 to S-72, Pergamon Press, Inc., 1969; Marshall, "Growth of Silicon Carbide from Solution", Material Research Bulletin, Vol. 4, pages S-73 to S-84, Pergamon Press, Inc., 1969; and Griffiths, "Defect Structure and Polytypism In Silicon Carbide", Journal of Phys. Chem. Solids, Vol. 27, pages 257-266, Pergamon Press, Inc., 1966.
In the method of this invention, in contrast to prior art methods of forming silicon carbide tertiary compounds, such tertiary compounds are formed as a coating or a layer on a substrate by a diffusion process wherein silicon carbide is diffused into a transition series metal layer at temperatures ranging from about 1000.degree. to about 1300.degree. C.
U.S. Pat. No. 3,772,058 to Bloom describes a method for coating metal substrates with a transition metal followed by application of metal carbides, nitrides, silicides or carbonitrides upon the coated substrate (Col 4, lines 28-31). In a preferred embodiment of the Bloom process metal carbonitride, such as silicon carbonitride, is vapor deposited on the transition metal coated substrate at a temperature ranging from at least 400.degree. C. to about 1200.degree. C. (Col. 4, line 31-Col. 5, line 25).
U.S. Pat. No. 2,784,112 to Nicholson describes the coating of a metallic substrate with a layer of silicon carbide. The coating is applied by heating silicon, silicon carbide and an inert filler in a carbon monoxide or other carbonaceous atmosphere within a temperature range of 1200.degree. to 1400.degree. C. Thus, this patent also does not teach the preparation of a tertiary compound on a substrate by diffusing silicon carbide into a transition series metal layer.